Silicon-on-insulator (SOI) bipolar junction transistors (BJTs) are used in high speed applications because of their improved packing density due to reduced parasitic capacitance effects. However, compared to conventional bulk silicon BJTs, SOI BJTs exhibit heat dissipation issues because of the high thermal resistance associated with the trench isolation and buried oxides surrounding the transistor.
In high performance SOI BJTs, thermal instability at high currents and voltages is a primary reliability concern. From the reliability point of view, thermal issues are mitigated by defining the safe operating area (SOA) based upon the power dissipation capability of the device.
Traditionally, the SOA of a power BJT has been determined using a discrete transistor in a fixed single measurement mode, i.e., a voltage (VBE) controlled mode or a current (IBE) mode. In the voltage-control measurement mode, the BJT exhibits a destructive thermal runaway mode whereby temperature rise increases device current causing an abrupt decrease in Beta at a particular VBE, as shown in FIG. 1. FIG. 1 also shows that at a critical VBE (>0.8V), current increases induce a rise in temperature that further increases current leading to destructive thermal runaway. In the voltage control method, the device's SOA is determined by the VBE and JE (emitter current density) that result in thermal runaway. VBE and JE decrease as VCE increases.
In the current controlled mode for determining BJT SOA, increases in emitter current (IE) cause a decrease in VBE, indicative of a negative device resistance region. This negative resistance region occurs at high applied power, as shown in FIG. 2. However, the transistor typically remains undamaged during fixed current measurements since there is no spontaneous feedback between temperature and current. In the emitter current controlled method, the device's SOA is determined by the VBE and JE that result in negative resistance. As in the case of the voltage control method, VBE and JE decrease as VCE increases.
As discussed above, the standard SOA extraction methods use the same current density onset of thermal runaway and the negative resistance with VCE bias to determine the SOA, as shown in FIG. 3. This SOA, however, merely indicates the onset of the device's thermal runaway or negative resistance regime, but does not assure that the BJT in steady-state is in a thermally stable operating region. For example, at the operating point P in FIG. 3, with an applied JE of 100 μA/μm2 and a VCE of 8V, the BJT does not display negative resistance at high current but deviates significantly from the low VC curve in FIG. 2. A temperature rise at operating point P results in a decrease of VBE by 0.04V from point A and an increase of IC by 2.5 mA, which is greater than 100% increase in IC (˜1.5 mA) at point B in FIG. 3.